This...

And/or might depend on how quickly you're firing your groups (didn't mention that...)

And/or... since you noticed your first 3 were 3/4", then threw the 4th or 5th shots, you might be shooting at your bullet holes, rather than shooting at the same POA. I only mention that since you did mention the group size of the first shots compared to the last shots. I make it a habit to never look at my POI when firing for groups until I'm done, that way I can keep myself disciplined to aim for the same POA without distraction. It's just human nature to let yourself subconsciously shoot towards your other bullet holes - kinda like your back of your mind thinks "the rest of those shots hit over there, I should aim over there..."

But yeah, I'd be grabbing a match or magic marker and checking my bullet jump.